Mobile terminals that are in accord with the latest 3GPP2 CDMA2000-1x EVDV standards (RevD) support high-speed uplink data transmission that allow partial retransmission of a transmitted data packet at the physical layer unlike previous releases which performed retransmission decisions at higher layers that incurred a greater latency. In accordance with these latest standards, high speed data transmission from the mobile terminal takes place over a Packet Data Channel (PDCH), which is code division multiplexed with a Packet Data Control Channel (PDCCH) and a Pilot Channel (PICH) for uplink transmission to the base station. At the mobile terminal, Cyclic Redundancy Check (CRC) bits are attached to a data packet that is to be transmitted. For example, a 16-bit CRC is appended to each data packet. The resultant combined data and CRC bits are turbo encoded and then interleaved, the latter to combat a correlated fading channel. The interleaved bits are then reorganized into N subpackets, where N is the total number of transmissions allowed for one data packet, which is three in accordance with the afore-noted standards. Each subpacket takes part or the entire data from the output of the interleaver. The rules for forming the subpackets for each data packet are known to the mobile terminal and to the base station if the data packet size is known. All subpackets associated with one data packet have the same size and can be overlapped. For one data packet, each transmission to the base station consists of one of the subpackets, which are consecutively numbered with a subpacket ID number (SPID) from 0 to N-1. If the mobile terminal does not receive an acknowledge message (ACK) from the base station, transmission by the mobile terminal continues one subpacket at a time until an ACK is received in response to a transmitted subpacket, or all N subpackets have been transmitted, whichever occurs first. If an ACK is not received after all N subpackets have been transmitted, the mobile terminal's higher layer then decides whether to resend the packet in smaller size packets or a same size packet again, subpacket-by-subpacket as was just done, or to just drop its attempt to transmit the packet.
In transmitting a packet to the base station, the mobile terminal first transmits subpacket number 0, which is modulated and code-division-multiplexed (CDM-ed) with the PICH and the PDCCH, with proper levels relative to the PICH. As will be discussed, the PDCCH transmitted with the PDCH includes information needed by the base station receiver to properly decode the subpacket. This information includes the SPID, the size of the input data packet, and an indication of a PDCH-to-PICH power ratio. At the base station, the received code-division-multiplexed signal is despread and demodulated to generate soft symbol metrics for the PDCH. If SPID=0, then the generated soft symbol metrics are passed through a deinterleaver and a turbo decoder to reform the bits within the transmitted data packet and its associated attached CRC bits. A CRC check is then performed on the decoded data bits to determine whether there is a match between the CRC calculated from those decoded data bits and the attached decoded CRC bits. If the CRC calculated from the received decoded data bits matches the decoded CRC (a CRC pass), it is assumed with high probability that the data bits have been received and decoded accurately. If there is not a match (a CRC fail), then it is assumed that the received data bits are not the same as the transmitted bits in the input data packet. The pass or fail result of the CRC is passed to the downlink ACK channel processing. If the CRC is a pass, then an ACK is sent to the mobile station; otherwise no ACK is sent to the mobile terminal.
If the mobile terminal detects an ACK from the base station over the downlink ACK channel, it considers the transmission to have been successful and processes the next data packet to be transmitted from the higher layer. If the mobile terminal does not receive an ACK, then it does not receive a new data packet from the higher layer and transmits the next subpacket, which has SPID=1, from the original transmitted data packet. At the base station this next received subpacket is despread and demodulated to generate soft symbol metrics. The soft symbol metrics are then subpacket-combined with the soft symbol metrics previously received for the subpacket having SPID=0. If subpacket SPID=0 and subpacket SPID=1 have overlapping data bits, accumulation of the soft symbol metrics is performed on the soft symbols corresponding to the overlapped bits. After combination, the resultant metrics are then deinterleaved and turbo-decoded. As before, a CRC check is performed on the resultant decoded bits. If the CRC results in a pass, the base station sends an ACK to the mobile terminal; otherwise the base station is silent. If a CRC fail results again, the mobile terminal transmits its last remaining subpacket, which has SPID=2, to the base station and the process is repeated again. This time the subpacket combining at the base station undoes the packet formation at the mobile station plus the combining, if necessary, over subpackets SPID=0, 1 and 2.
As afore-noted, in accordance with 3GPP2 standards, the maximum number, N, of subpacket transmissions for one data packet is three. Thus, if a CRC fail still results after the third subpacket transmission, the higher layer will decide whether to abandon transmission of this data packet or to transmit it again. In either case, the next data packet received from the higher layer, be it this same data packet being provided for retransmission the same data in smaller size packets or a totally different and new packet, is treated as a new packet at the physical layer.
As previously noted, the PDCCH carries information needed by the base station to properly recover the transmitted subpacket on the PDCH and from that or multiple subpackets, the transmitted data packet. In the 3GPP2 system, subpacket formation is dependent on the input data packet size. In addition, the maximum number of transmissions of subpackets per data packet is a fixed parameter in the 3GPP2 standards. As noted above, that number is three. In addition an indication of the PDCH-to-PICH power ratio is required for turbo decoding. In 3GPP2 standards, there are only two possible PDCH-to-PICH power ratios for one data packet size. Further, in order to properly decode a subpacket, its SPID needs to be known to enable the receiver to properly combine a de-segmented packet with the one or two previously received subpackets if its SPID is 1 or 2, respectively. Thus, in order to properly detect the PDCH and recover the transmitted data packet, the following information needs to be associated with each received PDCH: the data packet size; the subpacket SPID; and the PDCH-to-PICH power ratio. As is specified in the standards, this information is carried by the PDCCH in order to assist PDCH processing and is transmitted simultaneously with the PDCH subpacket whose information it is carrying and with which it is code-division-multiplexed.
At the base station receiver, the PDCCH carrying this needed information is demodulated and decoded prior to processing of the PDCH to enable the receiver to properly detect the PDCH and recover the transmitted data packet. Using the PDCCH to provide these parameters, however, has several disadvantages. Firstly, using the PDCCH contributes to the total interference level within a cell and therefore reduces the uplink capacity. Secondly, in order to achieve a certain frame error rate (e.g., an order of magnitude smaller than a PDCH target frame error rate), the PDCCH needs to be transmitted at a sufficiently high power level, which further degrades uplink capacity. If the PDCCH is not transmitted at a high enough power level, then an error in detecting the transmitted SPID, for example, can have a deleterious effect on the latency in recovering the PDCH. For example, if the base station detects the PDCCH and it indicates an SPID of 0 and a CRC fail results in decoding the PDCH, the base station will not send an ACK and will wait to receive the next subpacket from the mobile terminal. If the PDCCH associated with that next received subpacket indicates an SPID of 2, an error has occurred somewhere because the next subpacket should have an SPID of 1. In fact, the only way that the PDCCH for the next received subpacket could properly indicate an SPID of 0 would be if the mobile terminal had erroneously detected an ACK when base station had in fact sent nothing, and the mobile terminal in response to that detected ACK had transmitted the first subpacket of the next data packet. The probability, however, of detecting an ACK when the base station had in fact sent nothing is low if the downlink ACK channel is reliable. Thus, when an SPID is received in error, the transmitted information on the PDCCH has likely been corrupted. The base station in detecting the received PDCCH, however, only looks at the information contained in the currently received PDCCH and not simultaneously at previous ones and thus is unable to detect that something is in error. Thus, when it receives a PDCCH with an SPID of 2, it will incorrectly combine the soft metrics in the currently received subpacket with the soft metrics in the two previously received subpackets, resulting in a CRC fail on the reconstructed data packet. As a result, extra latency is added to the recovery of the PDCH since the data packet will likely have to be retransmitted in its entirety again. In order to have a negligible impact on system throughput, the probability of incorrectly receiving critical information in the PDCCH should be an order of magnitude smaller than the PDCH target error rate. To achieve this, the PDCCH power needs to be at a sufficiently high level, which as aforenoted, results in a degradation in uplink capacity.
A methodology of detecting the uplink packet data channel that is less reliant on receiving a high-powered supplemental packet data control channel is thus desirable.